I would go see the contractor, look him in the eye, and decide whether his explanation is bs or not. There are good reasons for topping your slab, but it's pretty unusual, and I would definitely be wary, especially if the contractor didn't mention to you why they were doing it. It's possible that the slab was cracked extensively due to poor workmanship, which may result in further cracking of the new concrete, hence aesthetic damage and possible water intrustion. Ask him! If you suspect him of 'evildoing', you can hire a testing firm to do some remote sensing, possibly using a GPR that may confirm your suspicions, maybe even in civil court. Good luck.